As a conventional communication terminal, there is proposed a technique in which a terminal that is to transmit data performs interference detection on each frequency channel and determines a frequency channel used for transmitting the data (for example, see Patent Literature (PTL) 1).
As another conventional communication terminal, there is also proposed a technique in which a terminal that is to transmit data compares interference levels and amounts of data transmitted to other terminals located within a communication range, and determines a frequency channel used for transmitting the data according to a comparison result (for example, see PTL 2).
As another conventional communication terminal, there is also proposed a technique in which a backoff time (hereafter also referred to as “backoff value”) is controlled according to a type of data to be transmitted, thereby determining timing to transmit the data (for example, see Non Patent Literature (NPL) 1).
As another conventional communication terminal, there is also proposed a technique in which communication is performed using a hopping pattern generated by an adaptive frequency hopping function so as to exclude any channel that can lead to degradation in communication quality (for example, see NPL 2).
FIG. 48 is a diagram showing an example of frequency channel determination by the communication terminal described in PTL 1.
As shown in FIG. 48, a system includes terminals 500 and 501.
The terminal 500 determines a frequency channel for communicating with the terminal 501.
First, the terminal 500 scans all frequency channels, and measures received signal power to measure interference (Step S2101). It is assumed here that the number of frequency channels is five.
Next, the terminal 500 determines, from a scan result, frequency channels that enable desired signal reception, and sets priorities of the determined frequency channels in ascending order of undesired signal level (Step S2102).
It is assumed here that CH1 to CH4 are frequency channels that enable desired signal reception, where CH2, CH4, CH1, and CH3 are given descending priorities in this order.
The terminal 500 then transmits information of the priority order to the terminal 501, and switches to frequency channel CH2 of the highest priority (Step S2103).
Upon receiving the information of the priority order from the terminal 500, the terminal 501 switches to frequency channel CH2 of the highest priority (Step S2103).
The terminal 501 then transmits a test packet to the terminal 500, in order to check whether or not the terminal 501 is synchronized with the terminal 500 on frequency channel CH2 (Step S2104).
Upon receiving the test packet, the terminal 500 transmits a response to the terminal 501.
Upon receiving the response, the terminal 501 checks whether or not the terminal 501 is synchronized with the terminal 500, from information included in the response. It is assumed here that the terminal 501 is synchronized with the terminal 500.
Since the terminals 500 and 501 are synchronized with each other on frequency channel CH2, the terminals 500 and 501 establish a communication link on frequency channel CH2 and perform data communication (Step S2105).
In the case where the terminal 501 is not synchronized with the terminal 500 on frequency channel CH2, the terminal 501 transmits a test packet on frequency channel CH4 of the next highest priority, in order to check whether or not the terminal 501 is synchronized with the terminal 500 on frequency channel CH4.
As described above, in PTL 1, whether or not the terminals are synchronized with each other is checked in order of the frequency channels of descending priorities, and communication is performed using the channel on which the terminals are synchronized with each other, thereby avoiding interference or collisions.
FIGS. 49A to 49D are diagrams showing an example of frequency channel determination by the communication terminal described in PTL 2.
As shown in FIG. 49A, a system includes terminals 502, 503, 504, and 505 for transmitting and receiving data.
The terminal 502 determines a frequency channel used for communicating with the other terminals 503 to 505.
The terminal 502 stores the number of packets transmitted to each terminal in a predetermined period. FIG. 49B shows the number of packets transmitted from the terminal 502.
The terminal 502 also stores an interference level on each frequency channel. FIG. 49C shows an interference level of each terminal on each frequency channel, which is stored in the terminal 502.
The terminal 502 calculates a weighted-average interference level for each frequency channel using the information in FIGS. 49B and 49C. FIG. 49D shows a calculation result of the weighted-average interference level for each frequency channel. Here, frequency channel CH2 is lower in interference level than frequency channel CH1. Accordingly, the terminal 502 determines CH2 as the frequency channel used for communicating with the other terminals 503 to 505.
As described above, in PTL 2, the frequency channel used for communication is determined based on, as priority information, the amount of data transmitted to each terminal, thereby avoiding interference or collisions.
FIGS. 50A and 50B are diagrams showing an example of backoff control by the communication terminal described in NPL 1.
FIG. 50A shows an example of data types and backoff values in the IEEE (Institute of Electrical and Electronic Engineers) 802.11e standard. A higher priority corresponds to a smaller backoff value, contributing to a greater opportunity of data transmission.
FIG. 50B shows an example of backoff control on voice data which is higher in priority and background data which is lower in priority.
Suppose voice data and background data occur at time T1. The communication terminal randomly selects a value from 3 to 7 as a backoff value for the voice data, based on the information in FIG. 50A. It is assumed here that the value 4 is selected. The communication terminal also randomly selects a value from 15 to 31 as a backoff value for the background data, based on the information in FIG. 50A. It is assumed here that the value 19 is selected.
At time T2, the backoff of the voice data ends and the communication terminal transmits the voice data. Meanwhile, the backoff of the background data is suspended until time T3 at which a predetermined time elapses from when the transmission of the voice data ends.
At time T3, the communication terminal resumes the backoff of the background data.
As described above, in NPL 1, backoff control is performed according to the data type, thereby avoiding interference or collisions.
In NPL 2, an adaptive frequency hopping function is defined in order to avoid interference on a wireless LAN and the like using the same frequency band, in Bluetooth®. According to this function, a hopping pattern is generated from a plurality of provided frequency channels so as to exclude any channel that can lead to degradation in communication quality, and communication is performed using the generated hopping pattern. A control apparatus determines frequency channels used for communication, and notifies a terminal of the determined pattern.
As described above, in NPL 2, the control apparatus generates the hopping pattern from which any channel that can lead to degradation in communication quality is excluded and performs communication using the generated pattern, thereby avoiding interference or collisions.